Method for densifying a transmitter and receiver network for mobile telephony

ABSTRACT

A method for densifying a transmitter and a receiver network for mobile telephony. The network includes base stations that each have three cells that are mutually spaced at 120° intervals about the base station. At least four different base stations have mutually different frequency sets in the cells, and base stations of different frequency sets are placed in a repetitive pattern, a so-called 4/12-frequency pattern. A first densification is achieved by causing a center base station that has a first frequency set, and that is located within a generally rhomboidal pattern of base stations, to change its frequency set to the same frequency set as the base stations located at the corners of the rhomboidal pattern. Additional base stations are positioned within the rhomboidal pattern, and the frequency sets of the additional base stations are selected so that the frequency or frequencies of their cells will differ from the frequency or frequencies of the cells of the nearest base station.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for densifying a transmitterand receiver network for mobile telephony.

Although the present invention can be applied to different types ofmobile telephone systems, it will be described in the following withrespect to a GSM mobile telephone system.

2. Description of the Related Art

The capacity of a mobile telephone system is restricted by interferencesbetween different transmitter and receiver units. Each transmitter andreceiver unit includes an antenna for transmitting and receiving signalsto and from a mobile telephone. Communication is effected on differentpredetermined frequencies, i.e. channels.

Two different types of interference must be taken into account whenplanning an antenna network, particularly in municipal environments.Interference between two channels on one and the same frequency is onetype of interference. In the case of the GSM system, this type ofinterference means that the carrier signal must be stronger than theinterference signal by more than 9 dB. When this criterion is notfulfilled, speech quality is poor and there is a risk of the call beinglost, i.e. broken-off.

The other type of interference is caused by the carrier signal beingdisturbed by a closely adjacent channel. The different permittedchannels are numbered in the GSM system. For instance, channel 72 may bedisturbed by either channel 71 or 73. In the case of this type ofinterference, the carrier signal must not be weaker than a closelyadjacent channel by more than 9 dB in the GSM system.

When planning a network of antennas operating on different channels, itis necessary to take the aforesaid interferences into account. Thisapplies primarily to municipal environments, in which the antennas areplaced relatively close together so as to make achievable a high trafficintensity with regard to the number of simultaneous calls.

There are a number of different accepted models on which a network canbe based. The majority of these models include so-called three-sectorsites, meaning that a base station is equipped with three directionalantennas, normally 60-degree antennas, the directions of which aremutually spaced by 120 degrees. Each antenna is supplied with one ormore channels and forms a so-called geographic cell. The antennasbelonging to a base station are supplied with different frequencies. Thebase stations are positioned in accordance with a pattern, in which thecells form an hexagonal configuration. Which channels are transmitted inwhich antennas is determined by the frequency pattern chosen.

A typical frequency pattern is a 4/12-pattern. In this case, allavailable frequencies are used once on four base stations includingtwelve cells. Positioning of the base stations and the cell frequenciesare repeated in a repetitive pattern with the cells forming saidhexagonal configuration, so that each cell that has one particularfrequency will be spaced as far as possible from an adjacent cell thathas the same frequency. In other words, this requires at least twelvechannels are needed to obtain a 4/12-pattern with one channel per cell.Each cell can be supplied with two channels, provided that twenty-fourchannels are available.

When capacity is deficient, a denser frequency pattern can be chosen.One such frequency pattern is a 3/9-pattern, meaning that all availablefrequencies have been used once on three base stations which includenine cells. Thus, in order to increase capacity in comparison with a4/12-pattern with twenty-four channels, twenty-seven channels are usedwith each cell being equipped with three channels. There is a great dealof uncertainty with regard to the function of a 3/9-pattern, because ofthe serious risk that interference problems will occur with subsequentpoor speech quality.

A further pattern, namely a 2/12-pattern, has been described. In thispattern, each base station has six sectors and the channels end up inaccordance with a given pattern. The 2/12-pattern is highly prone tointerference problems.

When needing to increase the capacity of a mobile network, a number ofdifferent measures can be taken. The first step in this regard is to adda channel, provided that further channels are available. In the case ofa 4/12-pattern, the access to thirty-six channels would mean that threechannels can be used with each cell.

Provided that twenty-four channels are used in a 4/12-pattern, capacitycan be increased by switching to a 3/9-pattern without needing to buildnew base stations. A 3/9-pattern, which requires twenty-seven channels,cannot be used to the full when only twenty-four channels are available.However, this means that the frequencies are repeated more frequently,which leads to quality impairment.

Another method employed in this regard involves the use of microcells.Microcells are small base stations of limited range, placed on the wallsof buildings for instance, about 5-10 meters above street level. A largenumber of microcells is able to relieve a superordinate network, i.e.the standard base stations. One drawback with microcells is that theycover only a small area, which results in a variation in signal strengthon the part of a user who moves quickly between the cells, since largevariations in signal strength occur when turning round street corners.Another drawback resides in the cost of each microcell. The indoorcoverage afforded by a microcell is also poor.

The present invention solves the problem of markedly increasing thecapacity of the network within a desired geographical area, with alimited number of available channels.

SUMMARY OF THE INVENTION

The present invention thus relates to a method for densifying atransmitter and receiver network for mobile telephony. The networkincludes base stations which each have three cells whose directions aremutually spaced by 120 degrees, wherein a base station associated withthree cells operates on mutually different channels, i.e. frequencies,at least four different base stations have mutually differentfrequency-sets in the cells, so that each of the cells of the four basestations operates at different frequencies. Base stations are so placedin relation to each other that the cells form an hexagonal pattern, withbase stations of different frequency-sets being placed in a repetitivepattern, i.e. a so-called 4/12-frequency pattern. Base stations areplaced generally equidistantly from one another along two or moregenerally parallel and straight lines, wherein each alternate basestation in a first line has a given first frequency-set and each otherbase station has a given second frequency-set. Each alternate basestation has a given third frequency-set along a second line and eachother base station has a given fourth frequency-set along the secondline. By virtue of the hexagonal pattern the base stations along oneline are displaced in relation to the base stations on an adjacent lineby a distance corresponding generally to half the distance between twobase stations lying along one and the same line. A first densificationis achieved by causing a center base station that has a firstfrequency-set and is located in a generally rhomboidal pattern whosecorners consist in the four base stations that are located nearest thecenter base station and have mutually the same frequency-set, i.e. asecond frequency-set, to change its frequency-set to the samefrequency-set as the base stations located in the corners of saidrhomboidal pattern. Further base stations are arranged inwardly of theconfines of said rhomboidal pattern. The frequency-sets of respectivefurther base stations are selected so that the frequency or frequenciesof their cells will deviate from the frequency or frequencies of thecells of the nearest base stations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference toexemplifying embodiments thereof and also with reference to theaccompanying drawings, in which

FIG. 1 illustrates a cell structure according to a 4/12-pattern;

FIG. 2 illustrates part of the cell structure in FIG. 1 in larger scale,in which densification is effected in a first stage in accordance withthe invention; and

FIG. 3 illustrates part of the cell structure in FIG. 1 in larger scale,in which densification is effected in a first and a second stage inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a conventional 4/12-pattern of the aforesaid kind.The reference numeral 1 identifies a base station, each of which issurrounded by three hexagonal cells 2, 3, 4. The cells thus form theaforesaid hexagonal pattern. The numerals 1 to 12 shown in the centersof the different cells refer to the channel group, or frequency-set,transmitted in respective cells. Each channel group may consist of onefrequency or of two or more frequencies, depending on the number offrequencies available to the net operator.

The present invention relates to such a network for mobile telephony.The network is thus comprised of a large number of base stations 1, eachhaving three cells 2, 3, 4, the directions of which are mutually spacedby 120 degrees. The three cells of a base station operate on mutuallydifferent channels, i.e. frequencies. The network includes at least fourdifferent base stations with cells that have mutually differentfrequency-sets in the cells, so that each of the cells of the four basestations will operate on a frequency that is different from that of theothers. The base stations 1 are positioned relative to one another suchthat the cells 2, 3, 4 in the network form an hexagonal pattern, withbase stations having different frequency-sets being placed in arepetitive pattern, i.e. a so-called 4/12-frequency pattern.

The base stations are placed generally equidistantly from one anotheralong two or more generally parallel and straight lines. Such lines arereferenced 5 and 6 in FIG. 1. Each alternate base station along thefirst line 5 has a given first frequency-set and each other base stationalong that line has a given second frequency-set. Each alternate basestation along the second line 6 has a given third frequency-set and eachother base station along that line has a given fourth frequency-set. Asa result of the hexagonal pattern, the base stations that lie on a firstline 5 are displaced in the longitudinal direction of the line inrelation to the base stations on an adjacent second line 6 by a distancewhich corresponds essentially to half the distance between two basestations that lie on one and the same line.

It has been said in the aforegoing that the base stations are spacedgenerally equidistant from one another along two or more essentiallyparallel and straight lines. It has also been said that the mutualdisplacement of base stations along adjacent lines correspondsessentially to half the distance between two base stations that arelocated on one and the same line. FIG. 1 illustrates an ideal4/12-pattern. In practice, a base station consists of a mast whichcarries antennas. It will be understood that it can be difficult, orquite impossible, particularly in densely built-up areas, to erect thebase stations in an ideal pattern, due to inaccessibility to buildingsand the lack of ground on which to place the base stations. Thepositions of the base stations in a virtual network will thereforediffer more or less from the ideal positions shown in FIG. 1.

According to the invention, a first densification of the network isachieved by causing a center base station 10 having a firstfrequency-set G and located in a generally rhomboidal pattern 7 whosecorners are defined by the four base stations 11-14 that are locatednearest the center base station and that have mutually the same secondfrequency-set R, to change its frequency-set G to the same frequency-setR as that of the base stations 11-14 at the corners of said rhomboidalpattern.

The four different frequency sets of the base stations are referenced G,R, B and S in FIG. 1. In addition to base stations having thefrequency-set R and located in the corners of the rhomboidal pattern,base stations 23-26 (see FIG. 2) are also found on respective pairs ofopposite sides of the rhomboid. These have the frequency-sets S and B,respectively.

Naturally, each rhomboid in the 4/12-pattern formed by base stationsthat have mutually the same frequency-set can be chosen to densify thenetwork within the rhomboid.

According to the invention, further base stations 15-22 are arrangedwithin the rhomboid 7 as shown in FIG. 2, which shows the rhomboid ofFIG. 1 in larger scale. The frequency-sets G, R, B and S of respectivefurther base stations 15-22 are selected so that the frequency orfrequencies of their different cells will differ from the frequency orfrequencies of the cells of the base stations nearest thereto.

It has surprisingly been found that a 4/12-network can be densified inthis way without incurring the interference problems mentioned in theintroduction. By changing the frequency-set of the center base station10 to the frequency-set of the base stations located in respectivecorners of the rhomboid, it is possible to choose the frequency-sets forthe further base stations which differ from the frequency-set of thecenter base station 10 and the corner base stations 11-14 and whichdiffer from the nearest base stations 15-26, including the additionalbase stations 15-22.

In speech communication, the cell of each base station will cover ageographic area which is smaller than the area covered by each originalcell, within which area the base station concerned will be the strongestbase station and dominate over the cells of surrounding lying basestations.

Thus, as a result of this densification, a larger number of basestations, and therewith cells, will be available for communicationwithin the densified area.

The mutual positions of the further base stations and their positions inrelation to the original base stations is not absolutely critical, andcan be varied. The number of further base stations can also be varied.

According to another preferred embodiment of the invention, however, thefurther base stations 15-22 are eight in number. The further basestations 15-22 are arranged within the rhomboid 7 in a rhomboidalpattern which is concentric with the rhomboid 7 and which has a sidelength which is essentially half of the side length of the largerrhomboid. Thus, in FIG. 2, the base stations 15, 17, 19 and 21 definethe concentric smaller rhomboid.

According to this embodiment, the frequency-set of respective furtherbase stations 15-22 in the concentrical rhomboidal pattern is the sameas the frequency-sets in a rhomboid formed by the original basestations, this rhomboid being displaced from and with its sides parallelwith the first-mentioned rhomboid 7 and having a center base stationwhich has said second frequency-set, namely the frequency-set R that thecenter base station has. Such a rhomboid is formed, for instance, by theoriginal base stations 10, and 36-38 as seen in FIG. 1. All of thesebase stations have originally the frequency-set G.

In the case of this embodiment, the smaller rhomboid is oriented so thatthe additional base stations 15, 17, 19 and 21 in the corners of thesmaller rhomboid have the first frequency-set, i.e. the frequency-set Gthat the base station 10 had originally.

A typical distance between the original base stations is about 1,000meter. As a result of the aforedescribed densification, the distancebetween the base stations in the densified area will be about 500 meter.

FIG. 3 further illustrates a highly preferred embodiment of theinvention in which the network is further condensed or densified. FIG. 3illustrates the region concerned in FIG. 2. This further densificationaround the first-mentioned center base station 10 is achieved by causingthe base station to revert back to its original first frequency-set G.In addition, eight additional base stations 28-35 are arranged inwardlyof the aforesaid smaller rhomboid, i.e. the rhomboid defined by the basestations 15, 17, 19 and 21. This further densification of the network isachieved in accordance with the same principles as those applied withthe first densification.

The further densification is thus achieved in a rhomboidal pattern whichis concentric with the smaller rhomboid and which thus becomes thesmallest rhomboid. The smallest concentric rhomboid has a side lengthwhich is essentially half the side length of said smaller rhomboid.

The frequency-set of respective additional base stations placed in thesmallest concentrical rhomboidal pattern is the same as thefrequency-sets in a rhomboid containing the original base stations, thisrhomboid being displaced parallel with, or coinciding with, the firstmentioned rhomboid 7 and having a center base station with the firstfrequency-set G.

The smallest rhomboid is orientated so that the additional base stations28, 30, 32 and 34 in the corners of the smallest rhomboid have saidsecond frequency-set R.

It has surprisingly been found that this high degree of densificationalso avoids the interference problems mentioned in the introduction,despite the fact that the distance between the additional base stations15-21 and 28-35 in the aforedescribed example is as short as about 250meters. According to one preferred embodiment of the invention, thecenter base station and the additional base stations grouped around thecenter base station have a transmission power which is essentially thesame as the transmission power of remaining base stations. This meansthat interferences between two mutually adjacent base stations that havethe same frequency-set will be effectively avoided because each cell isstrong within its own area. The network will also provide highlyeffective indoor coverage in built-up municipal areas.

According to one preferred embodiment of the invention, densification inan area outside and adjacent to the first-mentioned rhomboid 7,extending between the original base stations 11-14, is achieved in thesame way as that described above inwardly of a rhomboid of the same sizeas and bordering on the first-mentioned rhomboid. Naturally, the networkin any area whatsoever located outside the first-mentioned rhomboid canbe made denser in the aforesaid manner.

It has been said in the aforegoing that the relative positions of thebase stations with regard to each other and with regard to the originalbase stations can be varied and that the number of additional basestations can also be varied.

However, the aforedescribed preferred embodiments are very important,since a first densification of the network and also a seconddensification of the network in accordance with the aforegoing providesa highly dense network in which interference problems are avoided.

According to one preferred embodiment of the invention, one or more ofthe center base station and the further base stations groupedtherearound, and also other base stations are caused to transmit andreceive signals on tilted antennas where the lobe points obliquelydownwards, so as to limit the range of the respective antennas.

This embodiment can be applied when disturbances would otherwise occur,for instance due to two cells being too close together because theantennas cannot be positioned in any other way. For instance, it may bedifficult to choose optimal positions for the base stations in a denselybuilt-up area.

The embodiment can also be used in those instances when one or two ofthe additional base stations cannot be placed anywhere at all, forinstance due to the presence of waterways in municipal built-up areas.

The densification of an existing network according to a 4/12-pattern hasbeen described in the aforegoing. It will be obvious, however, that theinventive method can be applied equally as well to densify conventional4/12-patterns when building a completely new network.

Although the invention has been described above with reference to anumber of exemplifying embodiments thereof, it will be understood thatmodifications can be made with respect to the transmission strength ofindividual base stations or cells, and with regard to their positions,as imposed by practical circumstances.

The invention shall not therefore be considered limited to theaforedescribed and illustrated embodiments thereof, since variations andmodifications can be made within the scope of the following Claims.

What is claimed is:
 1. A method for densifying a transmitter and receiver network for mobile telephony, said method comprising the steps of: providing a network of base stations which each have three cells whose directions are mutually spaced by 120 degrees, wherein the three cells associated with a given base station operate on mutually different frequencies and wherein the network includes at least four different base stations that have mutually different frequency-sets in the respective cells, so that the cells associated with the at least four base stations each operate at different frequencies; positioning the base stations in relation to each other so that the cells form an hexagonal pattern with base stations of different frequency-sets being placed in a repetitive pattern generally equidistantly from one another along at least two spaced, generally parallel and straight lines, wherein each alternate base station in a first line has a given first frequency-set and each intervening base station in the first line has a given second frequency-set, wherein each alternate base station in a second line has a given third frequency-set and each intervening base station in the second line has a given fourth frequency-set so that by virtue of the hexagonal pattern of the cells the base stations along one line are displaced in relation to the base stations on an adjacent line by a distance corresponding generally to half the distance between two base stations lying along one and the distance between two base stations lying along one and the same line and so that two consecutive base stations in the first line and two adjacent consecutive base stations in the second line define corners of a generally rhomboidal outer pattern, and a center base station within the generally rhomboidal pattern and having a first frequency-set and wherein the corner base stations each have the same second frequency-set; changing the frequency-set of the center base station to the same frequency-set as that of the corner base stations; providing additional base stations inwardly of the generally rhomboidal pattern; and selecting the frequency-sets of the additional base stations so that the frequencies of their cells differ from the frequencies of the cells of the nearest base stations.
 2. A method according to claim 1, wherein the additional base stations include eight additional base stations arranged in a generally rhomboidal inner pattern which is within and concentric with the generally rhomboidal outer pattern such that the generally rhomboidal inner pattern has a side length which is half of a side length of the generally rhomboidal outer pattern; and the step of placing the additional base stations within the generally rhomboidal inner pattern pattern so that they have a respective frequency-set which is the same as the frequency-sets in a generally rhomboidal pattern that is displaced from and parallel with the generally rhomboidal outer pattern and having a center base station with the second frequency-set, wherein the additional base stations at the corners of the generally rhomboidal inner pattern have the first frequency-set.
 3. A method according to claim 2, including the steps of causing the center base station to switch its frequency-set back to the first frequency-set; and providing eight additional base stations within the generally rhomboidal inner pattern in a rhomboidal pattern which is concentric with the generally rhomboidal inner pattern to define an innermost generally rhomboidal pattern that has a side length which is half the side length of the generally rhomboidal inner pattern, wherein the eight additional base stations defining the innermost generally rhomboidal pattern have a respective frequency-set which is the same as the frequency-sets in a generally rhomboidal pattern that is displaced from and parallel with the generally rhomboidal outer pattern or coinciding with the generally rhomboidal outer pattern and that has a center base station with the first frequency-set, wherein the eight additional base stations that define corners of the innermost generally rhomboidal pattern have the second frequency-set.
 4. A method according to claim 1, wherein the center base station and the base stations grouped therearound have a transmission power which is essentially the same as the transmission power of other base stations in the network.
 5. A method according to claim 1, wherein the base stations transmit and receive signals with tilted antennas wherein an antenna lobe points obliquely downward to limit the range of the respective antennas.
 6. A method according to claim 1, wherein the distance between base stations is about 1,000 meters; and wherein the shortest distance between two base stations subsequent to densifying the network is about 250 meters.
 7. A method according to claim 1, including the step of densifying an area outwardly of and bordering on the generally rhomboidal outer pattern and extending between the base stations defining the generally rhomboidal outer pattern and positioned inwardly of a generally rhomboidal pattern of the same size and adjoining the generally rhomboidal outer pattern. 